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→Jumping to a Variable Location
Puppydrum64 (talk | contribs) m (→Jumping to a Variable Location: Added an example use case for JP (HL)) |
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The incredibly misleading <code>JP (HL)</code> will copy the value in the <code>HL</code> register pair directly to the program counter. You would think based on the parentheses that it looks up the value stored at the memory location in <code>HL</code>, but it doesn't! This is the fastest jump command the Z80 has.
<code>JP (HL)</code> can be used to create a "trampoline," which lets you indirectly call a subroutine, provided that subroutine doesn't take <code>HL</code> as an argument. (Otherwise you'll get some unwanted results.) To do this, you need to know the address of the subroutine you wish to call, load it into HL, then <code>CALL</code> any address that contains either <code>JP (HL)</code> or the byte 0xE9. (<code>JP (HL)</code>'s bytecode is 0xE9). Once you do this, execution will effectively "call
<lang z80>
ld hl,foo ;rather than directly specifying "foo", you would normally retrieve it by indexing a table.
;This was done only to keep the example as simple as possible.
;If you're going to load it as a constant into HL you're better off just CALLing it outright.
call Trampoline
;execution will return here, with HL = address of "foo" and A = 0.
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